Today's digital image display systems are designed to perform various video signal processing tasks. These digital image display systems include computer display systems, as well as many of today's television systems.
One video signal processing task is scaling, where the number of pixels per row is increased or decreased (horizontal scaling) or the number of rows is increased or decreased (vertical scaling). Various algorithms have been devised to add or subtract the pixel data required for scaling.
A problem with many vertical scaling algorithms is that the scaled image lacks sharp horizontal edges. For example, where an image has a black-to-white horizontal edge, if a new row of data is generated by interpolation, that new row might result in a grey row between the two rows that previously defined the edge.